Engines use various types of emission control devices to treat emissions in exhaust gas. The devices may use various types of materials to treat emissions, including precious metals and other catalysts. In some types of devices, the materials may have the ability to chemically or physically store oxidants, such as oxygen and/or NOx, during certain operating conditions. For example, some devices store oxidants during excess oxygen conditions. Likewise, some devices may release and/or convert stored oxidants under stoichiometric or oxygen deficient conditions.
Under some conditions, it may be desirable to generate a rich exhaust gas mixture to release and/or convert stored oxidants in an emission control device coupled to an engine's exhaust. One approach that attempts to reduce NOx in a catalyst is described by U.S. Pat. No. 6,729,126. In this approach, a variable valve control system adapted for changing the opening and closing timing of an exhaust valve is controlled so as to temporarily place the exhaust valve in an open position for a period of time before a moment of initiation of an exhaust stroke of the cylinder and then place the exhaust valve in the open position again during the exhaust stroke. Such operation is used when an exhaust gas emitted from the corresponding cylinder is desired to contain a reducing component for reducing NOx.
However, the inventors herein have recognized a potential disadvantage with such an approach, at least under some conditions. For example, it can be difficult to determine how much fuel and/or air is actually exhausted during the first opening of the exhaust valve, and thus the combustion air-fuel ratio may be too rich or too lean. Such variation can also result in inappropriate ignition timing, which can increase exhaust emissions and reduce fuel economy.
In one approach, at least some of the above disadvantages may be overcome by a method for operating an internal combustion engine having at least a first and second cylinder. The method comprises, during at least one operating condition, operating the first cylinder to receive directly injected fuel, mix said fuel with fresh air inducted past an intake valve of the first cylinder, combust said mixture, and exhaust at least a portion of said mixture past an exhaust valve of the first cylinder; and concurrent with said operation of the first cylinder, operating the second cylinder to hold an intake valve of the second cylinder closed for at least a cycle of said second cylinder, receive directly injected fuel, and then exhaust at least a portion of said fuel past an exhaust valve of the second cylinder to generate unburned reductants in an exhaust of the engine.
In this way, it is possible to provide reductant during cylinder deactivation operation while reducing any interruption of the combustion air-fuel mixture of the remaining cylinders carrying out combustion.
In another example, advantages may be achieved by a method for operating an internal combustion engine having at least a first and second cylinder. The method comprises operating with said second cylinder in a cylinder deactivation mode and said first cylinder carrying out combustion; and during said operation, directly injecting fuel into said second cylinder and temporarily opening an exhaust to provide unburned hydrocarbons to an exhaust of the engine.
Again, such operation enables the ability to generate unburned hydrocarbons in an engine's exhaust during cylinder deactivation while reducing any interruption of the combustion air-fuel mixture or spark timing errors of the remaining cylinders carrying out combustion.
Note that there are various types of direct injection systems, including high and low pressure systems, cylinder head injectors, side wall injectors, and various others.